Statine (4S-amino, 3S-hydroxy, 6-methylheptanoic acid) is of pharmaceutical interest as a key intermediate in the preparation of inhibitors of proteases such as pepsin and renin. It is disclosed as compound No. 8652 at page 1258 of The Merck Index, 10th edition (1983). Its structure is: EQU (CH.sub.3).sub.2 CHCH.sub.2 CH(NH.sub.2)CH(OH)CH.sub.2 CO.sub.2 H.
It has been demonstrated that pepsin inhibitors containing the unnatural 3R, 4S isomer of statine exhibit as much as a thousand-fold decrease in activity when compared to their counterparts prepared from the naturally occuring 3S, 4S isomer. See Liu, W.; Smith, S.; Glover, G.; J. Med. Chem. Vol. 22, No. 5 577-579 (1979, CA 90:161943h); Kawai, M.; Boparai, A.; Bermatowicz, M.; Rich, D.; J. Org. Chem., 48, No. 11, 1876-1879 (1983, CA 98; 215995Z); and Rich, D. H.; Son, Eric, T. O.; J. Med. Chem. 23, No. 1, 27-33 (1980, CA 52413F). For these reasons it has become desirable to prepare Boc-3S, 4S statine in large quantities for synthetic purposes. Its structure is: EQU (CH.sub.3).sub.2 CHCH.sub.2 CH(NHR)CH(OH)CH.sub.2 CO.sub.2 H
wherein R is C(O)OC(CH.sub.3).sub.3.
Initially, 3S, 4S statine was obtained by the purification of the hydrolysates of a number of pepstatins of pyridomycin, which yielded the natural isomer. However, the scale of these preparative procedures has been increased to only a modest level.
Chiral and diastereomeric statines can be made by a variety of techniques. The chiral methods suffer from various disadvantages. Often, they are multi-step procedures having moderate to poor yields, requiring exotic reagents, which in most cases do not yield a sufficiently high enantiomeric excess or purity to avoid a purification step to remove the unwanted isomer or other contaminants. The diastereomeric procedures, while being synthetically efficient, share with the chiral procedures the necessity for a purification step to remove the unwanted isomer.
Previously, such purification was accomplished by chromatographic methods which are not trivial, in that the Rf values of protected statine isomers are very similar. The selection of various derivatives of, or protecting groups, for the desired product(s) can increase or decrease the difficulty of this separation. See U.S. Pat. No. 4,397,786; Rich, D.; Son, E.; Bopari, A.; J. Org. Chem., Vol. 43, No. 18, 3624-3626 (1978. CA 89: 147704r); Rague, B.; Fehrentz, J.; Guegan, R.; Chapleur, Y., Castro, B.; Bull. Soc. Chem. Fr., 107-8 Pt. 11 230-2 (1983, CA 100; 210370e); and Gesellchen, P.; Univ. of Wisconsin, Madison, Wis., Diss. Abstr. Int. B 38(8) 3703-4, (1978), CA 88; 152951a).
The separation of isomers of Boc-statine by fractional crystallization has only been cited twice in prior literature. In one reference, the separation of 3S, 4S Boc statine from the 3RS, 4S Boc statine mixture was set out. However, it was later determined by another investigator that the product was still a mixture of isomers. That investigator obtained the pure isomers by chromatography. See the Rich et. al. article (cited above) and Steulmann, R.; Klostermeyer, H.; Liebigs Ann. Chem., 2245-2250 (1975, CA 84: 106020f.)